Linear fluorescent lights are often used to light classrooms, office buildings and doctors' offices. The fixtures all appear the same, but there is no real "standard" for all lighting applications. Within the linear fluorescent family are many other classifications like size, color and style.
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The size of linear fluorescent lamps refers to both its length and diameter. Linear fluorescents are generally referred to in a few different sizes with T8 and T5 being some of the most common. The "T" generally stands for "tube," although many manufacturing companies have their own meaning for it. The number immediately following represents the diameter measured in eighths of an inch. T5s are skinny tubes with ends measuring 5/8 of an inch in diameter. T8s are slightly larger tubes with ends measuring 8/8--or one inch--in diameter.
Although the main difference between T8s and T5s is their size, they also come in different wattages. T8s generally start around 17 watts and normally reach about 42 watts, with the standard being 32 watts. A 32-watt T8 lamp would normally be listed as F32T8.
T5 lamps vary widely in wattage from about five watts to 54 watts, with 17-watt and 54-watt lamps being the most common. F54T5s are widely used in place of T8s in certain applications, but because of their size and capacity, 54-watt lamps are specially made and labeled as "HO" or "high output." This means that the wattage is higher than normal for that size of lamp, so it has been specifically manufactured to handle the excess energy.
A ballast is basically a regulator for fluorescent lights. Because fluorescent technology uses gases mixed with electricity to create ultraviolet light, they would keep mixing and the light would intensify until it eventually burned out. The ballast not only creates the initial pulse of electricity to light the gases and phosphors, but it also regulates the electricity while the lamp is on, keeping it running at the correct wattage and temperature.
Although T5 and T8 lamps are very similar and are often used in the same applications, they must have ballasts that are manufactured specifically for not only their lamp type (T8 or T5) but also for their wattage. A 5-watt T5 will not run on a 54-watt T5 ballast, and vice versa. The same goes for T8 technology. Before changing out any lamp types or fixtures, make sure to have the correct ballast associated with your lamp type.
T8s are primarily used as overhead lighting in ceiling fixtures. They are used in commercial buildings and even homes. Kitchen lights, for example, are often long fixtures with a wrap lens that houses two or four 4-foot T8 lamps.
T5s, although sometimes used in overhead fixtures like industrial high-bays, are primarily used in under-cabinet or accent lighting. Often seen in offices and cubicles as the light bulbs underneath desk cabinets, they are also used in display cases at retail shops.
Compact and Circular Fluorescents
Because T8 and T5 refer specifically to the size of the bulb's tube, fluorescents of all shapes can be confused. Circular tube lights, compact fluorescents and U-Bend lamps are also referred to with a T5 or T8 size; however, they do contain specific characters in their nomenclature to differentiate between linear fluorescents and different shapes. Compact fluorescents are usually listed with "CF," and U-bend lamps are generally named with an "FB" for "fluorescent: bent."
Although the different types are measured in T8 and T5 sizes, "T8" and "T5" used alone refer to the linear fluorescent bulbs.